Add native windows CI configuration

Description

@aevri has been working on support for using buildstream on windows natively.

The following changes add in a test that will be run to check that the currently-implemented behaviour does not regress.

Changes proposed in this merge request:

• Adds a tests-win32-master test that installs buildstream and checks that bst help and bst init work.
• Adds a tests-win32-non-master test that does the same, but has to be started manually.

This merge request, when merged, will resolve #1163 (closed)

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